This invention relates to a papermaking fabric, and more particularly to a two-ply warp two-ply weft papermaking fabric having auxiliary weft yarns incorporated in the papermaking side of the fabric.
Numerous requirements have been heretofore requested in connection with papermaking fabrics. For example, (1) the problems of paper quality itself such as imparting smoothness to the surface of paper, preventing the occurrence of wire marks, and ensuring thorough mixture of cellulosic fibers and the problem of the papermaking retention, (2) the improvement of resistance of the fabric to wear and the extension of service life of the fabric, and (3) the increase of rigidity and textural stability of the fabric and the insurance of the ability of the fabric to allow high drainage capability are particular subjects of desired requirements which have been made in the past. In many respects, these desired requirements are interrelated. Generally, the requirement of (1) mainly concerns a problem regarding the quality of the papermaking side of the fabric, the requirement of (2) mainly concerns a problem regarding the running side construction of the fabric, and the requirement of (3) concerns the problem relating to the entire fabric.
Heretofore, for the fulfillment of requirement (1), many proposals have been made for the one-ply warp two-ply weft yarn papermaker fabric. However, no device has ever been developed for perfectly fulfilling the requirement of (2), i.e. the improvement of the resistance of the papermaking fabric to wear. The highest technical level is only enough to prevent the warps of the papermaking fabric against wear by exposing the weft yarns of the fabric on the running side to abrasion.
In recent years, however, the upward trend of the operating speed of the paper machine, the ratio of filler in the fabric, and the demand for neutral papermaking has reached a point where the resistance of the papermaking fabric to wear poses an important problem.
Generally, from the viewpoint of the textural stability of the fabric while in service and the extension of the service life of the fabric, the running face side weft yarns of the fabric are desired to perform the role of resisting wear.
When the warps are worn, the fabric naturally suffers loss of tensile strength and elongation of size. When the wear continues until the warps are cut, the fabric directly breaks and its service life is terminated.
For the purpose of improving the resistance of the fabric to wear, an attempt has been made heretofore to use polyamide yarns capable of resisting wear for the running side weft yarns of the papermaking fabric. This attempt promises no epochal effect because it resides exclusively in utilizing the quality of a material to be used instead of altering the construction of a woven fabric itself. The papermaking fabric which uses polyamide type yarns is at a disadvantage in exhibiting a deficiency in textural stability.
Another attempt has been made to use yarns of a large diameter for the running side yarns of the papermaking fabric. This modification has contributed to improving the resistance to a certain extent. However, many problems have arisen from the practical point of view such as disturbing the balance between the warps and the weft yarns, impairing the crimping property of yarns, and creating a tendency for the occurrence of wire marks.
An idea of increasing the numbers of warps and weft yarns forming the fabric and their densities may be conceived for precluding the occurrence of wire marks on the paper produced. To realize this idea, the warps and the weft yarns severally require decreases in diameter.
In the well-known one-ply warp two-ply weft yarn fabric which is now in popular use, however, such decreases of diameters result in degrading such properties of the fabric as resistance to wear, rigidity, and textural stability.
When the diameters are increased conversely for the purpose of improving the fabric with regard to resistance to wear, rigidity, and textural stability, the fabric is compelled to sacrifice the smoothness of surface and inflicts a wire mark on the paper to be produced. Thus, the increases or decreases of the diameters in question entail a contradictory problem.
An attempt is now under way to solve the problems mentioned above with a fabric which is obtained by forming a papermaking side fabric and a running side fabric with several different warps and weft yarns and joining the two fabrics into a one-piece papermaking fabric with the aid of binding yarns.
To be specific, the papermaking side fabric is formed with high density by using warps and weft yarns both of a small diameter, and the running side fabric is formed with high resistance to wear by using warps and weft yarns both of a large diameter.
This attempt, however, has not necessarily brought about a fully satisfactory result. In the parts of binding in which binding yarns and papermaking side warps intersect, since the binding yarns draw the papermaking side fabric toward the running side and consequently give rise to recesses in the surface of the papermaking side fabric, the marks of these recesses are transferred onto the paper being actually produced on the papermaking fabric and eventually inflict a wire mark on the produced paper.